Fat as a metabolic fuel = P & E notes

Question 1

State advanatges of using fat as a metabolic fuel

Answer 1

. Energy to mass ratio - double the energy per gram dry weight than glycogen
. Low hydration levels due to hydrophobic nature - low mass.
Energy dense and inert

Use as an energy source does not interfere with body function- unlike the use of protein supplies which involve muscle breakdown

Can be readily mobilised

Ideal energy source for tissues with high energy demand
Fatty acids are more reduced than glucose

There are large stores of fat in the human body

Question 2

State some disadvantages of using fat as a metabolic fuel

Answer 2

. Slow energy release - large number of metabolic processes before it can enter the TCA cycle
. Cannot be transported across cell membranes - so requires intracellular lipases to mobilise TAGs as fatty acids -
Cannot be transported directly across cell membranes but instead requires lipases to mobilise TAGs as fatty acids and an extracellular lipase is needed to take up circulating TAGs as fatty acids
Insoluble in water so need specialised transport systems

Question 3

Outline process of lipolysis

Answer 3

TAG to DAG + FA - ATGL
DAG to MAG + FA - HSL
MAG to glycerol + FA - MAG lipase
Enzymes HSL = Horomone Sensitive Lipase, MAG lipase = Monoacylglycerol lipase, ATGL = Adipose Triacylglycerol lipase
Substances - TAG = triacylglycerol
DAG = diacylglycerol
MAG= monoacylglycerol
FA = fatty acid

Question 4

Role of albumin

Answer 4

• Fatty acids are transported in blood by being bund to albumin

Question 5

Hormonal regulation of lipolysis
Adrenaline = … lipolysis
Insulin = …. lipolysis

Answer 5

stimulates
inhibits

Question 6

Lipid droplets are covered with ….
significance of this …

Answer 6

Perilipin = a protein which covers lipid droplets
Perilipin = controls adipocyte lipid metabolism - How ?
Perilipin = bound to an ATGL activator called CGI 58

Question 7

How does adrenaline stimulate lipolysis

Answer 7

Adrenaline = binds to GPCRs
. Adenylate cyclase converts ATP to cAMP which activates PKA
- Phosphorylates HSL & perilipin
. ATGL is activated because phosphorylation of perilipin causes CGI 58 to be dissociated from perilipin

Question 8

How does insulin inhibit lipolysis

Answer 8

Insulin stimulates phosphodiesterase because:
Insulin - stimultates phosphodiesterase which converts cAMP to ATP so activation of PKA is prevented
. Insulin = prevents activation of PKA - thus inhibiting lipolysis

Question 9

State the enzyme which catalyses the production of fatty acyl-CoA

Answer 9

Fatty acid = esterified with CoA by acyl-CoA synthetase

Question 10

Outline Carnitine Shuttle

Answer 10

Reason for Carnitine Shuttle = Fatty acyl CoA cannot cross the IMM
IMM = Inner Mitochondrial Membrane
CAT1 = Catnitine acyl transferase
CAT1 exchanges CoA for carnitine
Translocase shuttles fatty-acyl carnitine across the IMM in exchange for carnitine
CAT 2 = exchanges carnitine for CoA

Question 11

Elaborate on the control of carnitine shuttle

Answer 11

Control of carnitine shuttle is by regulation of CAT1 bymalonyl-CoA(unique intermediate in synthesis of FAs) whichinhibits CAT1

Produced from acetyl-coA by acetyl-coA carboxylase (ACC)

ACC mediated by whole body energy status

ACC activated byinsulin- synthesis

ACC inhibited byglucagon- oxidation

Fed state (after a meal) -> more insulin -> activation of ACC -> increased synthesis of FAs + decreased oxidation

ACC mediated by cellular energy status

ACC activated byATP- synthesis

ACC inhibited byAMP- oxidation

AMP dependent protein kinase inhibits ACC

Question 12

Where does B oxidation take place

Answer 12

In the mitochondrial matrix

Question 13

How do fatty acids enter the cytoplasm of hepatocytes

Answer 13

FA - enter the cytoplasm of hepatocytes either through simple diffusion or facilitated diffusion through FAT
FAT = fatty acid translocase

Question 14

CAT-1 = rate limiting enzyme of beta oxidation

Question 15

Diseases - involving CAT enzymes & Carnitine

Answer 15

Clinical relevance - carnitine deficiency

Lack of CATs

Muscle weakness + fatigue in prolonged exercise

Treatment = carnitine supplements

Question 16

Experimental Evidence

Answer 16

Experimental evidence

Malonyl Co-A inhibits oxidation of fatty acids to ketone bodies in homogenates (whole cell fragments) but not in mitochondrial fragments

Inhibits entry to the mitochondria

Malonyl CoA = inhibits Carnitine Acyl Transferase -1

Question 17

Where does B- oxidation take place
State the products of B oxidation

Answer 17

Location = mitochondrial matrix
Overall pathway = produces
acetyl-CoA - used in TCA
NADH & FADH - used in ETC

Question 18

One B oxidation = shortens the fatty acyl-CoA chain by ……

Answer 18

2 carbon atoms

Question 19

State the reactions of B oxidation

Answer 19

Reactions of β oxidation

FAD-dependent oxidation

Hydration

NAD+-dependent oxidation

Thiolysis

Question 20

State in detail the steps in Beta oxidation

Answer 20

Oxidation
Fatty Acyl CoA -> Trans Enoyl CoA &
FAD -> FADH2
Enzyme: Acyl CoA Dehydrogenase
Hydration
Trans Enoyl CoA + H20 -> Hydroxyacyl CoA
Enzyme: Enoyl CoA Hydratase
Oxidation
Hydroxyacyl CoA + NAD -> NADH + H+ + Ketoacyl CoA
Enzyme: Hydroxyacyl CoA Dehydrogenase
Thiolysis
Ketoacyl CoA + CoASH -> Fatty Acyl CoA (2 C shorther) + Acetyl CoA
Acetyl CoA = Krebs /TCA cycle
Fatty Acyl CoA = enters B oxidation cycle again

Question 21

Name 3 defects of fatty acid oxidation

Answer 21

MCAD deficiency
Carnitine deficiency
Jamaican vomiting sickness

Question 22

MCAD deficiency elaborate

Answer 22

MCAD deficiency means that medium chain length fatty acids build up as intermediates of beta-oxidation

They bind to carnitine to form carnitine conjugates which are then excreted in the urine resulting in carnitine deficiency

Rare genetic disorder

Onset in infancy with 25% sudden and unexpected death

50% have hypoglycaemia because cannot rely as much on fat stores for ATP synthesis

No or reduced ketogenesis on fasting

Symptoms are vomiting, lethargy, seizures, coma, death

Question 23

Carnitine deficiency

Answer 23

A relatively rare genetic disorder, 1/19000

Cardiomyopathy because the heart is very dependent on fatty acids

Fatty infiltration of organs because they are not completely oxidised by other organs

Muscle weakness because slow twitch fibres are heavily reliant on fatty acids as fuel

Hypoglycaemia as glucose stores are used in place of fats

Question 24

Jamaican vomiting sickness

Answer 24

Not a genetic defect, but a poisoning which inhibits an enzyme

Unripe fruit from the ackee tree contains a compound called hypoglycin A

When metabolised, this compound inhibits acyl-CoA dehydrogenase

This prevents beta-oxidation so the body has to rely solely on glucose

Leads to hypoglycaemia, coma and death

Question 25

Cytoplasmic fatty acids binding protein (cFABP) moves fatty acids to intracellular compartments as fatty acids are insoluble in the cytoplasm so have to bind to a soluble protein, this includes their transport to the mitochondrial membrane to move into the mitochondria for beta-oxidation

Question 26

The enzymes of fatty acid oxidation

Answer 26

The acyl-CoA dehydrogenase is different depending on the length of the fatty acid chain

VLCAD- very long chain acyl-CoA dehydrogenase is 18+ carbons

LCAD- long chain acyl-CoA dehydrogenase is 12-18 carbons

MCAD- medium chain acyl-CoA dehydrogenase is 6-12 carbons

SCAD- short chain acyl-CoA dehydrogenase is 4-6 carbons

Question 27

Unsaturated fatty acids

Answer 27

Contain a cis double bond which is converted to a trans bond by isomerase

Reductase then removes the double bond and makes the fatty acid a substrate for ongoing oxidation

Question 28

Very long chain fatty acids

Answer 28

Have 20-22 carbons so must first be shortened in the peroxisomes to 16-18 carbons (called peroxisomal beta-oxidation), then transferred to mitochondria for beta-oxidation

Peroxisomal beta-oxidation does not generate ATP, but instead hydrogen peroxide

Question 29

Odd chain length fatty acids

Answer 29

Beta-oxidation generates acetyl-CoA which has 2 carbons, but with odd chain length fatty acids, the last round of beta oxidation won’t do this so the last round will start with 5 carbons

The last cycle will generate 1 acetyl-CoA and 1 propionyl-CoA (3C) which can be metabolised to succinyl-CoA to enter the TCA cycle

Question 30

Branched-chain fatty acids

Answer 30

Start in the peroxisomes for alpha-oxidation, before they are transferred to the mitochondria for beta-oxidation

Question 31

Name the shuttle in
- fatty acid synthesis
- fatty acid oxidation

Answer 31

FA synthesis = CITRATE shuttle
FA oxidation = CARNITINE SHUTTLE

Question 32

Fat Facts

Answer 32

Fats give twice as much energy per gram when oxidised than glucose

Weighs less than glycogen because it is an anhydrous store because it is hydrophobic

6 times more efficient to store than glycogen

Question 33

Assimilation of dietary fats
Outline & Read about the process of
Assimilation, emulsification, absorption, packaging as chylomicrons

Answer 33

The large triglyceride droplet in the intestinal lumen is hydrophobic and inaccessible to pancreatic lipases

Bile salts are powerful detergents which breakdown the large lipid droplets into smaller droplets

Pancreatic lipases can now attack the droplets- they require a large surface area to operate

Lipases then breakdown TAGs into NEFAs and glycerol which spontaneously arrange into mixed micelles with a hydrophobic centre and a hydrophilic surface- these can be absorbed through the intestine wall

Triacylglycerols are enclosed in lipoprotein to form chylomicrons which consist of a phospholipid monolayer with apolipoproteins embedded which act as cofactors for enzymes and ligands for receptors

Chylomicrons are then released into the lymphatic system through exocytosis

Question 34

Transport and release of fatty acids

Answer 34

Chylomicrons travel in the lymphatic system because they are too big to enter capillaries

Lymphatic vessels drain into large veins so lipids bypass the liver

Lipoprotein lipase on the surface of endothelial cells break down chylomicrons- these are found on the surface of adipose and muscle tissue and releases NEFAs which are then taken up by the cells

Question 35

The role of insulin in fat distribution and storage

Answer 35

Insulin causes the body to build-up TAG stores as it causes GLUT4 recruitment which increases adipocyte intracellular glucose which can be stored as TAGs

Insulin is antilipolytic so prevents breakdown of fat stores and stimulates production of lipoprotein lipase to allow the breakdown of chylomicrons so the NEFAs can be stored

Insulin stimulates transcription of genes involved in fatty acid esterification